This invention relates to a system and method for fastening rivets and/or using process indicators to communicate to operators the stage of each rivet during a rivet setting cycle. In particular, the invention relates to a system and method that relies on sensors that are used as part of a feedback control system to achieve rivet setting tolerances by measuring in real-time or near-real-time the rivet's driven head (sometimes called the upset head or shop head) height while the control system also controls rivet gun operation and communicates the rivet driving-cycle stage to the rivet setting operator(s).
Riveting produces the strongest practical means of fastening airplane skins and substructure together. Although the cost of installing one rivet is small, installing the great number of rivets used in airplane manufacture represents a large percentage of the total cost of any airplane.
It should be first noted that the term “tolerance” is used broadly throughout this disclosure. Conventionally, the term tolerance signifies a plus or minus range of acceptance on a bell-shaped-curve distribution of samples with preferably the peak of the bell-shaped curve representing the optimum and the distribution of samples being bounded by a narrow band having upper and lower specification limits. The curve is used with a measure of standard deviation to quantitatively characterize defects by a measure of standard deviation or sigma value. In this disclosure, the term tolerance also sometimes refers to a specific value representing the optimum peak of the bell-curve (or very near peak, i.e., extremely tight tolerance). For example, “It is often difficult to consistently set rivets to meet tolerances but it is extremely difficult to consistently set rivets to an optimal tolerance.” In other words, a very tight tolerance being met consistently by a large data set having that is both accurate and precise also has a high sigma value.
Although this invention may be applied to special types of rivets, for purposes of clarity, this disclosure uses as an example conventional solid-shank rivets that comprise a manufactured head, a shank, a shank end, and a driven head. The driven head is formed by upsetting the rivet shank with a rivet gun or rivet driver while backing the shank with a bucking bar. The shank actually expands slightly while being driven so the rivet fits tightly in the drilled hole and the shank end deforms to produce a driven head. Fastened material is then held between the manufactured head and the driven head.
Where there is easy access to both sides of the work, the rivet-gun operator can sometimes simultaneously drive the rivet and back the rivet with a bucking bar; however, in most cases both a rivet-gun operator and a bucking-bar operator or bucker must work together to drive solid-shank rivets. The conventional procedure for driving rivets is as follows: (1) the rivet gun operator adjusts the air regulator which controls the air pressure and/or air flow to increase or decrease hitting force of the pneumatic rivet gun; next (2) the rivet gun operator inserts the rivet into the drilled hole, places the rivet set tool anvil face against the rivet and waits for the bucker; next (3) the bucker holds the bucking bar anvil face on the opposite end of the rivet; next (4) the rivet gun operator should “feel” the pressure being applied by the bucker through the rivet; and finally (5) the rivet-gun operator will start the rivet gun by pulling the trigger to release a short burst of rivet-gun blows and then stop the rivet gun when the rivet has been driven or set to be within a desired range of manufacturing specifications or tolerances. Forward-set rivets are formed when the set tool hammers on the manufactured head and the bucking bar backs the shank end of the rivet. Backset rivets are formed when the set tool hammers on the shank end of the rivet and the bucking bar backs the manufactured head of the rivet. It is to be understood that teachings of this disclosure apply to both forward-set and backset rivet driving methods and one skilled in the art could apply the teachings of one method to achieve another method.
Throughout the rivet setting process, both operators must hold their tools perpendicular or orthogonal to the work so the rivet is driven axially. The entire rivet setting process requires both skill and experience since the rivet-gun operator must determine rivet gun burst-length or blows needed according to variables such as manual forces applied by either bucker or gun operator, the rivet size being driven, the rivet gun design and air-flow or pressure settings and the mass of the rivet gun and bucking bars. These variables must be judged by the rivet-gun operator to time the length of the rivet driving stage needed to achieve desired rivet set tolerances.
Further, to communicate with each other, the rivet-gun operator and bucker conventionally use a tapping code to enable the bucker to communicate with the rivet-gun operator: one-tap on the rivet by the bucker means start or resume driving the rivet (resuming is often necessary when the rivet has been under-driven and has not reached tolerance); two-taps on the rivet by the bucker means the finished or set rivet was within satisfactory tolerance; three-taps on the rivet by the bucker means the rivet was improperly set and must be removed (this typically occurs when the rivet has been over-driven and can not be modified to achieve tolerance). Where verbal communication is possible, the rivet-gun operator typically announces “ready” when he is ready to begin riveting and waits for the bucker to likewise announce “ready” when he is ready to begin bucking and follows with a “good”, “drive more” or “not good” verbal report of the completed set rivet.
To achieve design strength, the driven head of a rivet must fall within an acceptable tolerance range; to inspect rivets, the bucker sometimes uses a gauge to measure the driven head-height or driven head-width after the rivet has been set. Often, however, to save time, the bucker only visually inspects the driven head to determine if it meets required tolerances. If the rivet has been under-driven leaving the head height too high, additional driving is needed (although due to work hardening of the rivet material, rivet holding strength for rivets driven in repeated driving stages is often reduced). Over-driven rivets require removal, which is a time consuming process that can often damage the work and sometimes requires using an oversized replacement rivet having a different setting tolerance. Over-driven rivets often blemish or bend the work, sometimes causing costly rework or irreparable damage.
The background art is characterized by U.S. Pat. Nos. 1,803,965; 2,354,914; 3,478,567; 3,559,269; 3,574,918; 3,933,025; 4,218,911; 4,566,182; 5,398,537; 5,953,952; 6,011,482; 6,088,897; 6,357,101; 6,363,768; 6,823,709; and 7,331,205; the disclosures of which patents are incorporated by reference as if fully set forth herein.
Although the conventional method of driving rivets described above has been effective for many years, there are some background art inventions that have unsuccessfully attempted to improve the process. What is needed is a rivet fastener system that overcomes the disadvantages of the background art; such a rivet fastening system is disclosed herein.